In general, in a wireless communication system, when a mobile terminal unit such as a cellular phone initiates communication with a base station unit, the mobile terminal unit may get access to the base station unit by using an access method called random access. In the random access, a mobile terminal unit transmits a communication establishment request including an identifier called preamble and the like to a base station unit through an RACH (Random Access CHannel), thereby establishing communication between the mobile terminal unit and the base station unit.
Specifically, in the random access, a mobile terminal unit transmits a communication establishment request to a base station unit through an arbitrary access slot out of multiple random-sequence access slots. Therefore, if multiple mobile terminal units transmit a communication establishment request through the same access slot at the same timing, there may be a random access contention between the mobile terminal units. In the event of occurrence of a random access contention, after waiting for a predetermined time, the mobile terminal unit again transmits the communication establishment request to the base station unit through an arbitrary access slot, thereby retrying to establish communication with the base station unit. Consequently, when a random access contention occurs, establishment of communication between the mobile terminal unit and the base station unit is delayed.
In recent years, to prevent such a delay in establishment of communication, there has been proposed a technology to constantly assign a fixed access slot to a particular mobile terminal unit. Specifically, a mobile terminal unit given priority for performing communication is assigned an access slot that the other mobile terminal units do not use. According to such a technology, random access by the particular mobile terminal unit is not in contention with random access by the other mobile terminal units, so there is no delay in the establishment of communication between the particular mobile terminal unit and the base station unit.    Patent Literature 1: Japanese Laid-open Patent Publication No. 2007-82259    Patent Literature 2: Japanese Laid-open Patent Publication No. 2009-112023    Patent Literature 3: Japanese Laid-open Patent Publication No. 08-154097
However, the above-described conventional technology has a problem that a wireless resource is constantly occupied by a particular mobile terminal unit. Specifically, in the conventional technology to assign a fixed access slot to a particular mobile terminal unit, the other mobile terminal units do not use the access slot assigned to the particular mobile terminal unit. As an access slot is a wireless resource, if the access slot is constantly occupied by a particular mobile terminal unit, which means the wireless resource is constantly occupied by the particular mobile terminal unit.